The field of the invention relates to films for packaging foodstuffs and other articles.
Certain films, though having excellent properties such as transparency, stiffness, and moisture barrier, have unacceptably high coefficients of friction which makes them difficult to utilize in automatic packaging equipment. Highly crystalline polypropylene film is one such film having the above-mentioned properties.
A number of approaches have been taken to improve the surface friction characteristics of films, including polypropylene films. One such approach is described in U.S. Pat. No. 3,176,021, and involves the inclusion of minor quantities of fatty acid amides into the polypropylene. U.S. Pat. No. 4,419,411, which is incorporated by reference herein, discloses a film having a base layer comprising polypropylene of high stereoregularity, the precursor resin of which contains an amide of a water-insoluble monocarboxylic acid, and a poly-olefin skin layer containing finely divided silica and a silicone oil. Some of the amide within the base layer blooms to the skin layer, thereby reducing the coefficient of friction to 0.25 or less at a temperature up to about 140.degree. F.
The use of various silicone oils has been found to provide satisfactory reductions in the coefficients of friction of a variety of films, thereby facilitating their use in automatic packaging machines. U.S. Pat. Nos. 4,652,489, 4,659,612, 4,692,379, 4,720,420, 4,734,317 and 4,764,425, which are incorporated by reference herein, all disclose sealable films which employ polydialkylsiloxane as a slip agent. U.S. Pat. Nos. 4,659,612, 4,692,379 and 4,734,317 provide methods for reducing the coefficient of friction of both sides of a film laminate by adding a silicone oil such as polydimethylsiloxane to one surface layer thereof, and then contacting this surface layer to the second surface layer of the film to transfer some of the oil thereto. The second layer may be corona or flame treated to enhance its receptivity to water-based coatings such as certain inks and adhesives prior to the application of the silicone oil.
Films having a layer incorporating polydialkylsiloxane are relatively difficult to treat using conventional methods, and the bond strengths of such films when laminated may also be unacceptably low. Corona or flame treating a film surface layer containing such a silicone oil reduces the heat sealability thereof and increases the coefficient of friction. The benefits of the silicone oil are accordingly lost. Metallized coatings applied to such a surface layer may not tend to adhere as well to such surface layers as compared to surfaces devoid of silicone oil.